This application is based on and hereby claims priority to German Application No. 10050371.3 filed on Oct. 11, 2000, the contents of which are hereby incorporated by reference.
The present invention relates to a device preferably operating in the temperature range of liquid air. The material, which inevitably has to be able to withstand mechanical load and/or temperature changes and embrittlement, of the component of the device which can be subjected to high mechanical loads must also have required ferromagnetic properties and/or a high magnetic conductivity for the intended magnetic flux guidance and/or flux amplification.
It is known from, e.g. Reed, Materials at Low Temperatures, pages 388/389, to use nickel steel with a nickel content of between 3.5 and 9% for components which can be subjected to mechanical loads for cryogenic applications, i.e. for the temperature range  less than 173 K. Examples of such components include tanks which are required for storing and transporting liquefied gases, such as for example liquid air. Nickel steel with a nickel content of 12% has also already been developed for temperatures below 77 K and especially for use in connection with liquid helium at 4 K. It is essential for these materials that the transition temperature from ductile to brittle be reduced to below the intended application temperature. The abovementioned reference by Reed also refers to additives which may be present in the nickel steel.
However, for the use of a material which corresponds to the invention, it is not only its temperature-dependent mechanical property which is of crucial importance, but also it is necessary for the material which is mechanically suitable for the temperature range mentioned also to have a ferromagnetic property.
The latter property is crucial, for example, when the material is to be used for magnetic flux guidance and/or flux amplification in machines and machine parts which include superconducting elements. Examples are contactless bearings which also include superconducting structural elements which hold a rotating machine shaft in a floating position without contact in a bearing shell by magnetic repulsion forces. In this context, reference is made, for example, to German Utility Model U 9403202.5 xe2x80x9cMagnetische Lagerungseinrichtung mit Hoch-Tc-Supraleitermaterialxe2x80x9d and to U.S. Pat. No. 5,777,420 xe2x80x9cSuperconducting Synchronous Motor Constructionxe2x80x9d.
Therefore, it is an object of the present invention to use a material which, at the low operating temperatures required, has sufficient strength, even in the long term, for the expected mechanical loads, e.g. does not become brittle at the low temperatures, and moreover has ferromagnetic properties for the purpose of magnetic flux guidance and/or amplification, for a component of the device which is of relevance in this context.
It was determined as a result of tests being carried out that a nickel steel X8Ni9, DIN 1.5662, US standard ASTM A353, A553 has ferromagnetic properties for the abovementioned purposes even at temperatures below 77 K. This is a temperature range which is preferably of relevance to the invention. Even at these low temperatures, this material has so little tendency to become brittle that it does not suffer any ductile fracture even under mechanical load. Nickel steel containing from 3.5 to 8% of nickel also has a sufficiently high ferromagnetic property for the above-mentioned applications to enable it to be used for elements for magnetic flow guidance and/or amplification. The same is also true of nickel steel containing 12 to 13% of nickel.
Not only does the mechanical strength of the material with ferromagnetic properties required in accordance with the object play a role in, for example, rotating machine parts, on account of the centrifugal forces which occur, but also reliably mechanical stable material is advantageous or even imperative for the encapsulation and holding of magnet with high-temperature superconductor coils. This is the case, for example, if the holder also acts as a flux-guiding means.